Natural rubber is a rubber with the best physical and mechanical properties. Crude rubber, rubber compounds and vulcanizates of natural rubber all have outstanding strength, elongation, and elasticity, etc. The outstanding properties of natural rubber largely benefit from the high molecular weight of natural rubber. The weight-average molecular weight of natural rubber is usually higher than 1 million. However, high molecular weight of rubber always results in poor processability; but natural rubber has favorable processability. The Mooney viscosity of rubber is an important index that characterizes the processability of rubber. If the Mooney viscosity is higher than 90, the processability is usually poor. The Mooney viscosity of natural rubber is usually approx. 60-70, which can fully meet the processing requirement, because natural rubber has unique molecular weight distribution. It is generally agreed that the molecular weight distribution of natural rubber is bimodal distribution, in which a high molecular weight peak endows outstanding physical and mechanical properties to natural rubber, and a peak or “shoulder” in the low molecular weight region provides an effect of plasticizing agent and endows favorable processability to natural rubber.
High-cis polyisoprene prepared through a synthesis process is reputed as “synthetic natural rubber” and is the best substitute for natural rubber, because its structure is similar to that of natural rubber. Rare earth catalysts have high catalytic activity, high cis selectivity, and high molecular weight, and are regarded as catalysts most suitable for preparation of polyisoprene. Though polyisoprene with high molecular weight can be prepared with rare earth catalysts, the molecular weight distribution of the polyisoprene is unimodal distribution and covers a very narrow range. For example, a rare earth catalyst and a method for preparation of polyisoprene with the rare earth catalyst are disclosed in Chinese Patent Application CN101045768A, and the rare earth catalyst is disclosed as consisting of the following components: aluminum alkyl, chlorides, conjugated diene, and neodymium carboxylate, and the mole ratio of the components is 5-30:1-4:5-20:1; the weight-average molecular weight of polyisoprene prepared with that method can be as high as 1.43 millions, but the molecular weight distribution is unimodal distribution and the molecular weight distribution index is not more than 3.0. The polyisoprene with high molecular weight in unimodal distribution has poor processability, can't be mixed homogeneously with auxiliary agents such as carbon black and sulfur, has adverse effect to the performance of final product, and may even have loose, separation, and poor viscosity phenomena, resulting in processing failure (see Collection of Articles on Rare Earth Catalyzed Synthetic Rubber, the Fourth Research Department of CAS Changchun Institute of Applied Chemistry, Science Express, 1980, p 365-370). To facilitate the processing of polyisoprene, usually the following two solutions are used: the first solution is to decrease the molecular weight of polyisoprene so as to ensure the processability of the polymer, at the cost of physical and mechanical properties; for example, a method of introducing aromatic hydrocarbons into the catalyst system to decrease the molecular weight of the polymer is disclosed in Chinese Patent Application CN1295087A, and a method of introducing 3d transition metallic organic compounds into the catalyst system to decrease the molecular weight of the polymer is disclosed in Chinese Patent Application CN1342718A; the second solution is to widen the range of molecular weight distribution of polyisoprene, and thereby synthesize a polyisoprene with wide and steamed bread shaped unimodal molecular weight distribution; however, such polyisoprene doesn't exhibit outstanding physical and mechanical properties and high processability simultaneously, and still has a gap to natural rubber in terms of physical and mechanical properties and processability.
Presently, the research in preparation of polymer with bimodal molecular weight distribution is commonly seen in the synthetic polyolefin resin domain. The methods include melt bending method, staged reaction method, and dual active center catalyst method; with the former two methods, it is difficult to obtain homogeneously mixed polymer products with bimodal distribution, and the preparation process is complex and results in high cost; relatively ideal polymer products with bimodal distribution can be obtained with the dual active center catalyst method (see CN101085818A). The research in preparation of polymer with bimodal distribution is rarely seen in the synthetic rubber domain. The Chinese Patent Application CN101085818A discloses a dual active center catalyst, which can be used to synthesize polydiene with bimodal distribution, wherein, the peak molecular weight of high molecular weight component fraction is 6.5×105-9.0×105, the peak molecular weight of low molecular weight component fraction is 1.0×105-2.2×105, and the content of polydiene in cis-1,4-structure is higher than 96%; in addition, the catalyst is disclosed as comprising neodymium carboxylate compounds, organo-aluminum compounds, halogen-containing compounds, and C6-C10 carboxylic acids. Owing to the fact that the macromolecular chains of polyisoprene will break under mechanical shearing action when polyisoprene is processed, for bimodal polyisoprene, the higher the molecular weight of the high molecular weight component fraction is, the better the performance of polyisoprene will be, provided that the processability is not affected adversely (the processability of rubber is usually poor if the Mooney viscosity is higher than 90). However, it is a pity that no significant advance has been made in the research for increasing the molecular weight of the high molecular weight component fraction in bimodal polyisoprene.
In addition, the content of polyisoprene in cis structure in polyisoprene compound is also an important influencing factor for the performance of the polyisoprene compound, besides molecular weight and molecular weight distribution. In the polyisoprene prepared with the rare earth catalyst disclosed in CN85102250A, CN1834121A, or CN101045768A, the content of polyisoprene in cis-1,4-structure is usually approx. 96%; therefore, the performance of the polyisoprene is inferior to the performance of polyisoprene prepared with titanium-based catalysts, in which the content of polyisoprene in cis-1,4-structure is higher than 98% (see Collection of Articles on Rare Earth Catalyzed Synthetic Rubber, the Fourth Research Department of CAS Changchun Institute of Applied Chemistry, Science Express, 1980, p 70-82). Even if the content of polyisoprene in cis-1,4-structure in polyisoprene compound is increased slightly, the performance of the polyisoprene compound will be improved significantly (see Collection of Articles on Rare Earth Catalyzed Synthetic Rubber, the Fourth Research Department of CAS Changchun Institute of Applied Chemistry, Science Express, 1980, p 255-265). A method for preparation of polyisoprene with content of cis-1,4-structure higher than 98% is disclosed in CN1479754A and CN101186663A, but the method requires a non-homogeneous catalyst and/or demanding harsh process conditions.